Demand for polarizers has surged with the diffusion of liquid crystal display devices (hereinafter referred to as LCDs). In a polarizer, a protective film or protective films are generally laminated on one or both sides of a polarizing layer having polarizing ability through an adhesive layer.
As a raw material for the polarizing layers, polyvinyl alcohol (hereinafter referred to as PVA) has been mainly used. PVA films are dyed with iodine or dichromatic dyes after uniaxial stretching, or stretched after dying, and further crosslinked with boron compounds, thereby forming polarizing films for the polarizing layers.
As the protective films, cellulose triacetate has been mainly used, because it is optically transparent and has low birefringence. Usually, the films are longitudinally uniaxially stretched, so that absorption axes of the polarizing films are longitudinally roughly parallel.
In the conventional LCDs, transmission axes of the polarizers are arranged, inclined at 45° to the longitudinal or lateral direction of images. Accordingly, in the stamping process of the polarizers produced in the roll form, stamping has been carried out in a direction inclined at 45° to the longitudinal direction of the rolls.
However, stamping in the direction at 45° generates unusable portions in the vicinity of ends of the rolls. In particular, large-sized polarizers have the problem of a low yield. Further, for the polarizers after lamination, it is difficult to recycle materials thereof, which introduces the problem of increased waste.
Birefringencial films are used by adhering them to the polarizers or the like forming the LCDs, for optical compensation such as coloring prevention and enlargement of the angle of visibility, and it is needed to set orientation axes of the birefringencial films at various angles to the transmission axes of the polarizers. Conventionally, a system of cutting longitudinally or laterally uniaxially stretched films by stamping out film thereof in such a manner that orientation axes thereof give specified angles inclined to the edge thereof, which has raised the problem of a low yield, similarly to the polarizers.
For solving this problem, some methods are proposed in which the orientation axes of polymer films are inclined at desired angles to film transferring directions. JP-A-2000-9912 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes that a plastic film is longitudinally or laterally stretched at speeds different from each other on the right and left to the stretching direction, while uniaxially stretching the film in a lateral or longitudinal direction different from the above-mentioned stretching direction, thereby inclining the orientation axis thereof to the above-mentioned uniaxial stretching direction. However, according to this method, when a tenter system is used, for example, it is necessary to give the difference in transferring speed between the right and left to the stretching direction. As a result, crease due to uneven stretching stress, wrinkles and local unevenness of film thickness caused thereby are developed, resulting in difficulty in obtaining a desired inclined angle (45° in the polarizer). When the difference in speed between the right and left is intended to be decreased, the stretching stage must be lengthened, which causes installation cost-to be greatly increased.
Further, JP-A-3-182701 proposes a method for producing a film having a stretching axis of any angle θ to a running direction of the film, which comprises holding the continuous film at both of its right and left edges with a plurality of pairs of holding points inclined at the angle θ to the running direction of the film, and stretching the film in the θ direction by means of each pair points with running of the film. However, this method also causes crease and wrinkles to be developed in the film, because of different running speed between the right and left edges of the film. For relieving this problem, the stretching stage must be lengthened, which causes installation cost to be greatly increased.
Furthermore, JP-A-2-113920 proposes a method for stretching a film in a direction diagonal to a longitudinal direction of the film, which comprises running the film while holding it between two rows of chucks running on a tenter rail arranged so that the running distances of the chucks within, a specified running section on both sides of the film are different from each other. However, this method is also unfavorable for the production of an optical film because crease and wrinkles are developed when the film is stretched in the diagonal direction.